Page 131 - Digital Electronics by harish

 Auxiliary Carry (AC): Auxiliary Carry flag is set (1) if there is a carry from
bit position D of result in the accumulator, otherwise it is reset (0). This flag
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is used for BCD operations.

 Parity (P) flag: Parity flag is set (1) if the result in the accumulator has even
number of 1s, otherwise it is reset (0).

 Carry (CY) flag: Carry flag is set (1) if the result of an arithmetic operation
results in a carry from bit position D , otherwise it is reset (0). This flag is
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also used to indicate a borrow condition during subtraction operations.

5. Instruction register

When an instruction is fetched from memory, it is stored in the Instruction
register. It is an 8-bit register. This resister cannot be used in the programs.

6. Instruction Decoder and Machine cycle encoding

This unit decodes the instruction stored in the Instruction register. It
determines the nature of the instruction and establishes the sequence of events
to be followed by the Timing and control unit.

7. General purpose registers

There are six 8-bit general purpose registers namely B, C, D, E, H and L
registers. B and C registers are combined together as BC register pair for 16-bit
operations. Similarly D and E registers can be used as DE resister pair and H
and L as HL register pair. The HL register pair is also used as memory pointer
(M-register) for storing 16-bit address in some instructions.There are two more
8-bit temporary registers W and Z. These registers are used to hold data during
the execution of some instructions. W and Z registers cannot be used in
programs.

8. Stack Pointer (SP)

Stack is a portion of memory (RAM) used as FILO (First In Last Out)
buffer. This is mainly used during subroutine operations. Stack Pointer is a 16-
bit register used as a memory pointer (16-bit address) for denoting the stack
position in memory. The Stack pointer is decremented each time when data is
loaded into the stack and incremented when data is retrieved from the stack.
Stack pointer always points to the top of the stack memory.

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